Biofuels have a long history ranging back to the beginning of the 20th century. As early as 1900, Rudolf Diesel demonstrated at the World Exhibition in Paris, France, an engine running on peanut oil. Soon thereafter, Henry Ford demonstrated his Model T running on ethanol derived from corn. Petroleum-derived fuels displaced biofuels in the 1930s and 1940s due to increased supply, and efficiency at a lower cost.
Market fluctuations in the 1970s coupled to the decrease in US oil production led to an increase in crude oil prices and a renewed interest in biofuels. Today, many interest groups, including policy makers, industry planners, aware citizens, and the financial community, are interested in substituting petroleum-derived fuels with biomass-derived biofuels. The leading motivations for developing biofuels are of economical, political, and environmental nature.
One is the threat of ‘peak oil’, the point at which the consumption rate of crude oil exceeds the supply rate, thus leading to significantly increased fuel cost results in an increased demand for alternative fuels. In addition, instability in the Middle East and other oil-rich regions has increased the demand for domestically produced biofuels. Also, environmental concerns relating to the possibility of carbon dioxide related climate change is an important social and ethical driving force which is starting to result in government regulations and policies such as caps on carbon dioxide emissions from automobiles, taxes on carbon dioxide emissions, and tax incentives for the use of biofuels.
Ethanol is the most abundant fermentatively produced fuel today but has several drawbacks when compared to gasoline. Butanol, in comparison, has several advantages over ethanol as a fuel: it can be made from the same feedstocks as ethanol but, unlike ethanol, it is compatible with gasoline at any ratio and can also be used as a pure fuel in existing combustion engines without modifications. Unlike ethanol, butanol does not absorb water and can thus be stored and distributed in the existing petrochemical infrastructure. Due to its higher energy content which is close to that of gasoline, the fuel economy (miles per gallon) is better than that of ethanol. Also, butanol-gasoline blends have lower vapor pressure than ethanol-gasoline blends, which is important in reducing evaporative hydrocarbon emissions.
Isobutanol has the same advantages as butanol with the additional advantage of having a higher octane number due to its branched carbon chain. Isobutanol is also useful as a commodity chemical and is also a precursor to isobutylene and isobutylene-derived fuels and chemicals. Isobutanol has been produced recombinantly in yeast microorganisms expressing a heterologous metabolic pathway (See, e.g., WO/2007/050671 to Donaldson et al., and WO/2008/098227 to Liao et al.). However, these yeast microorganisms fall short of commercial relevance due to their low performance characteristics, including low productivity, low titer, low yield, and the requirement for oxygen during the fermentation process. One of the primary reasons for the sub-optimal performance observed in existing isobutanol-producing microorganisms is the undesirable conversion of pathway intermediates to unwanted by-products.
Thus, there is an existing need to identify and reduce and/or eliminate the metabolic processes catalyzing the conversion of isobutanol pathway intermediates to unwanted by-products. The present inventors have addressed this need by providing recombinant microorganisms with reduced pyruvate decarboxylase (PDC) activity and reduced glycerol-3-phosphate dehydrogenase (GPD) activity.